1. Field of Invention
The present invention relates to a thermal printer having two thermal print head and platen roller sets disposed along the thermal paper transportation path for printing on both the front and back sides of double-sided thermal paper.
2. Description of Related Art
A thermal printer that prints on double-sided thermal paper having thermo-sensitive surfaces formed on both sides of the paper is disclosed in U.S. Pat. No. 6,784,906. The disclosed thermal printer has a first print head and first platen supported on a first arm and a second platen opposing the first print head and a second print head opposing the first platen supported on a second arm installed on the printer frame side. When roll paper is pulled from a roll of double-sided thermal paper and the first arm is closed to the second arm, the double-sided thermal paper is set between the first print head and second platen on the upstream side of the paper transportation direction and between the second print head and the first platen on the downstream side in the transportation direction, the first print head opposes the front side of the double-sided thermal paper, and the second print head opposes the back side of the double-sided thermal paper. The front and back sides of the double-sided thermal paper can therefore be printed simultaneously.
However, the thermal printer disclosed in U.S. Pat. No. 6,784,906 fails to solve the following problems.
First, when the first arm closes to the second arm, the first print head and first platen carried on the first arm are simultaneously pressed to the respectively opposing second platen and second print head. As a result, even if the first arm is closed while pulling on the double-sided thermal paper so that there is no slack, the first and second platens which are free at this time may rotate due to the tension of the double-sided thermal paper because the double-sided thermal paper is simultaneously held between the first print head and the second platen on the upstream side and the second print head and first platen on the downstream side, and slack results easily between the first and second platens. If slack develops, the double-sided thermal paper may become skewed while being conveyed. An operation to remove the slack is therefore necessary.
Second, the first print head and first platen on the first arm must be positioned to contact the second platen and second print head on the second arm during printer assembly, for example. This requires simultaneously positioning the two print head and platen sets, which makes positioning more difficult than when the first print head is positioned to the second platen and the first platen is separately and independently positioned to the second print head.
Third, because there are two sets of print heads and platens, repulsion to the pressure applied to press the first print head to the second platen and repulsion to the pressure applied to press the second print head to the first platen both work when the first arm closes. This requires applying twice as much force to the first arm as when there is only one print head and platen set and a single print head is pressed to the platen. A strong operating force is therefore required to overcome twice the repulsion force when closing the first arm, and the first arm can be easily deformed by this strong operating force. When the cover to which the first arm is attached is closed to the printer frame side and an assembly that latches at both widthwise sides are used, deformation of the first arm when the cover closes can result in only one side engaging in the closed position and the other side not completely engaging in the closed position. The possibility is therefore high that the cover may be latched on only one side.
Fourth, because two sets of print heads are always pressed against the opposing platens when the first arm is closed, the paper transportation load on the double-sided thermal paper passing therebetween is increased. Two sets of print heads are also always pressed against the double-sided thermal paper in the same way as when both sides are printed even if only one side of the paper is printed using one print head similarly to when single-sided thermal paper having a thermo-sensitive coating rendered on only one side of the thermal paper is printed. This creates the problem that printing in a manner that is optimal for printing on only one side is not possible.